A drive laser assembly for an EUV light source is known, for example, from US 2006/0192152 A1 and US 2009/0095925 A1. The radiation source of the drive laser assembly is used to generate so-called seed pulses, which are amplified in the optical amplifier or amplifiers to high laser powers of multiple kilowatts, possibly 10 kW or greater. The laser radiation amplified by the drive laser assembly is supplied via a beam guiding unit to a focusing unit, which focuses the laser radiation in a target region. A target material is provided in the target region, which passes into a plasma state upon the irradiation using the laser radiation and emits EUV radiation at the same time.
In the above-described drive laser assembly, a first laser pulse (pre-pulse) and chronologically successively a second laser pulse (main pulse) are typically generated by the radiation source and focused on the target region having the target material. The first laser pulse is to be used for the purpose of influencing the target material, for example, heating it up, expanding it, vaporizing it, ionizing it, and/or generating a weak or possibly a strong plasma. The second laser pulse is to be used for the purpose of converting the main part of the material influenced by the first laser pulse into the plasma state and generating EUV radiation at the same time. The first laser pulse typically has a significantly lower laser power than the second laser pulse. In the drive laser assembly of US 2009/0095925 A1, the same laser wavelength is used for the first laser pulse and for the second laser pulse.
It is also possible to use different wavelengths for the first laser pulse and for the second laser pulse, as described in WO 2011/162903 A1, in which a seed laser is used to generate the first laser pulse and a further seed laser having a different wavelength is used to generate the second laser pulse. The two laser pulses having different wavelengths are combined by means of a beam combiner to pass through one or more amplifiers and the beam guiding unit following the drive laser assembly along a common beam path.
In the above-described drive laser assembly, a reflection of the amplified laser radiation can take place, for example, at the target material, which can be provided in the form of tin droplets, for example. The reflection generated at such a droplet passes back in the optical amplifier or amplifiers and passes through the amplification medium provided therein, and therefore the reflection is also amplified in the optical amplifier or amplifiers. A weak reflection is also possibly sufficient to generate a power after the amplification in the amplification medium of the optical amplifier or amplifiers, which can damage the optical or possibly mechanical components in the amplifier assembly or in the beam path before the optical amplifier.
Using so-called optical isolators, which transmit laser radiation in only one direction and are also referred to as optical diodes because of this property, is known for suppressing reflected laser radiation. Such optical isolators can be arranged, for example, between a radiation source and an optical amplifier or also between two optical amplifiers. For example, installing an optical diode in each case between an injection seeding laser and a resonator and between a resonator and an amplifier is known from DE 41 27 407 A1.
In the above-described drive laser assembly, high laser powers of 500W or greater, of 1 kW or greater, and even of 10 kW and more can be generated during the amplification. At such high laser powers, the problem exists that the optical components used in conventional optical isolators possibly induce strong thermally-related aberrations, in particular astigmatism, and moreover can possibly be damaged by the laser radiation. Moreover, the problem exists that optical isolators generally cannot 100% suppress laser radiation that propagates in the undesired direction, and therefore at high laser powers, in spite of the use of optical isolators, the non-suppressed power component is sufficiently large that returning laser radiation is possibly generated in spite of the use of an optical isolator. This is additionally made more difficult in the present application because optical diodes, which are based on the principle of the polarizer or phase shifter because of their construction, can only suppress laser radiation having a specific phase jump or having a specific phase shift (for example, 180°). The value for this phase jump or for this phase shift will possibly not be maintained upon the reflection on a droplet, and therefore the laser radiation reflected on the droplet can also not be completely suppressed by the optical diode for this reason.
Eliminating harmful feedback reflections from a power amplifier in a laser master oscillator is known from EP 0 674 375 A2, by arranging a non-reciprocal (acousto-optical) frequency shift unit between the oscillator and the amplifier, which shifts the laser frequency by more than twice the bandwidth of the optical resonator of the laser oscillator.